NSW opposition claims WestConnex blowout

The NSW government is facing a potential cost blowout on construction of Sydney's new M5, with the opposition claiming a contractor is demanding an extra half a billion dollars to finish the project.

Internal government documents, obtained by NSW Labor, show a contractor for stage two of the controversial WestConnex project is asking for an additional $600 million due to "unforeseen planning approval requirements".

Opposition leader Luke Foley told reporters Premier Gladys Berejiklian must take ownership of the current infrastructure "mess" the government finds itself in.

Stage two of WestConnex, also known as the new M5, will provide twin underground motorway tunnels from Kingsgrove to the St Peters Interchange.

The release of the leaked documents comes as the government battles Spanish subcontractor Acciona in court over the firm's claim for an extra $1.2 billion to complete the delayed CBD light rail project.

The opposition claims the WestConnex dispute means the project won't be up and running until 2021 - a year behind schedule.

"This is nothing short of scandalous," Mr Foley said in a statement on Thursday.